I am using Avast Pro (of course, no complaints!)
I am also using IE8 on Vista 32.
i ask whether there is benefit, or detriment, to be had by installing Microsoft “Security Essentials” http://www.microsoft.com/security_essentials/ in addition to Avast Pro.
I wouldn’t suggest using that because having two running antivirus programs may cause problems. They’ll conflict with each other, so it’s worse than having just one. Hope this helps. 
completely unneeded…why use two AV ??? MSE is good too, but you must choose + if you install it on top of Avast it might conflict with it. MSE is a full antivirus solution, with a service running etc…meaning that even if you deactivate the resident protection, its drivers will be loaded and run processes in parallel with Avast services. Not good.
I agree, don’t ever run two AV’s. We have MSE on the Vista machine here and so far , so good. It’s simpler to use but the one thing I’m not real fond of is that it uses the Windows Update mechanisms to update it’s database.
Not to use with avast.
They could work for a while. Then will conflict and fight to the detection and for the files. It’s the same history of having to applications as residents trying to do the same.
You need to chose. The problems, when happen, is that one antivirus could avoid the detection of the other. Here two is NOT synonym of more secure.
If your going to use both you should turn off resident protection in MSE. Check my post here. You can then scan with MSE as a second opinion.
http://forum.avast.com/index.php?topic=59852.msg504804#msg504804
I was using Microsoft Security Essentials for awhile with Avast; but it slowed down my old computer to a crawl. So I uninstalled Microsoft Security Essentials and just started using Avast by it’s self. The computer started running faster with just Avast on the system.